1. Field of the Invention
This invention relates generally to data storage systems and more particularly to such systems having a wide write element and a narrow read element.
2. Description of the Prior Art
Direct access storage devices (DASD) or disk drives, store information on concentric tracks of a rotatable magnetic recording disk. A read/write head is moved from track to track in order to read and record the desired information. Typically, the head is positioned on an air bearing slider which flies above the surface of the disk as the disk rotates. In some proposed disk drives, the slider rides on a liquid film or bearing on the disk. A head suspension assembly connects the slider to a rotary or linear actuator which moves the head to different radial positions along the disk.
During operation of the DASD units, it is important that the head stay positioned over the center of the track on which it is writing or reading data. This has become increasingly difficult as the tracks have become smaller in width and are spaced closer together in order to increase the overall data density of the DASD. The head may be off center due to differences in thermal expansion of the different parts of the DASD, in accuracies in the head position actuator, imperfect axial rotation of the spindle motor which rotates the disk, or any of a number of other factors.
One solution to the head mispositioning problem is to move the head in incremental steps across the track and try to determine the actual center of the track. The references which discuss this include U.S. Pat. No. 4,485,418 issued Nov. 27, 1984; European Patent Application 0220382 published Jun. 5, 1987; IBM Technical Disclosure Bulletin, Vol. 35, No. 4B, p. 303, September 1992; IBM Technical Disclosure Bulletin, Vol. 29, No. 2, p. 586, July 1986.
Incomplete erasure of a previously recorded data track is another problem which occurs due to mispositioning of the head. When a new data field is written on a track, it overwrites the previous old data field. However, if the head is not properly centered on the track, then a slice of the old data may remain at the edge of the new data field. This may cause errors in reading.
One approach to solving this problem has been to provide erase bands between the data tracks. The head may have additional separate erase elements on either side of the write element in order to insure that the edge of each newly written field does not contain any data. Alternatively, the write element may be used to create the erase bands on either side of a newly written data field. Examples of this include U.S. Pat. No. 4,858,048 issued Aug. 15, 1989; U.S. Pat. No. 4,771,346 issued Sep. 13, 1988; U.S. Pat. No. 4,644,421 issued Feb. 17, 1987; Japanese application 03-157805 published Jul. 5, 1991; German patent application DE2654577 published Jun. 8, 1978; IBM Technical Disclosure Bulletin, Vol. 32, No. 10B, p. 90, March 1990; IBM Technical Disclosure Bulletin, Vol. 28, No. 1, p. 203, June 1985.
Magneto-resistive beads have recently been used in DASD devices. These heads contain separate write and read elements. The wide write element is a transducer element which is optimized for writing. The narrower read element is a magneto-resistive element which is optimized for reading. The magneto-resistive head allows smaller and more closely spaced data tracks to be written and read, thereby increasing the overall capacity of the DASD.
The inventors have discovered that because the magneto-resistive read element is much narrower than the write element, there is a good chance that a previously written field which has not been completely erased may be mistakenly read. The probability of reading an old data field is further increased by the need to hunt across the track width in some circumstances in order to successfully read a data field. This hunting only increases the chance than an old unerased data field at the edge of the track may be mistakenly read. The addition of erase bands on either side of the data track is not practical due to the close spacing of the tracks. The edge of a data track cannot be erased without also erasing a portion of a neighboring data track.
What is needed is a DASD system and method which does not need erase bands and which will not mistakenly read incompletely erased previously data fields.